This invention relates to a method and apparatus for protecting a steam-driven turbine from damage caused by moisture particles entering the turbine with the steam. More particularly, the herein invention concerns monitoring the temperature and pressure of the steam to determine that the necessary superheat level is present to assure that all the steam enters the turbine in a gaseous phase.
Presently, many steam turbines use superheated steam as their driving medium and energy source. It is well known that as steam conditions deteriorate and approach saturation, moisture or liquid steam condenses in the turbine stages. The presence of moisture droplets or particles result in erosion of the moving blades and to a lesser degree the stationary diaphrams and nozzles in the turbine. Significant water ingestion can additionally cause damage to thrust bearings, journal bearings, and seals. Replacements of these parts is an expensive and time-consuming repair.
The degree to which steam is superheated may be determined if both the temperature and pressure of the steam are known. With these two parameters, one only has to go to conventional steam tables or to a Mollier chart to verify that the quality of the steam is such that it is superheated, to verify the degree of superheat and to verify the temperature and/or pressure drop the steam may undergo before reaching saturation.
Generally, operators of turbines have a poor understanding of steam quality. Also, during operation, problems and system upsets to, for instance, to a boiler providing steam, occur so fast that the operators do not have enough time to go to a steam table or to a Mollier chart to determine if steam quality is no longer acceptable, and that the turbine should be shut down. Additionally, many plants do not have operators and thus have no moisture protection for the turbine should the steam quality change.
During cold start-up of turbines, the standard practice is to warm up the main steam headers supplying steam to the turbine inlet. This practice is utilized such that all the water in the header is changed to the gaseous state to verify that superheated steam will be provided to the turbine on start-up. This minimizes the amount of moisture induced into and conducted through the turbine. Many times operators fail to realize the importance of this procedure and start turbines with cold steam headers and/or saturated steam.
To overcome the shortcomings of the existing operating procedures, the subject matter in the herein application was developed. The present apparatus and method monitors the steam temperature and pressure at the inlet line to the turbine. The monitored temperature and pressure are then utilized to determine the degree of superheat and the saturation temperature to ascertain whether or not the turbine should be operated. If the turbine should not be operated, the supply of steam to the turbine is interrupted automatically without operator action. The device also acts as a permissive start-up device in that it requires superheated steam to be available at the turbine inlet before the turbine can be started.
Additionally, since the pressure of the incoming steam to the turbine inlet is monitored, the same control signals may be utilized to determine either a low pressure condition or an over pressure condition. The turbine may be shut down in a like manner upon either of these conditions being detected by closing the inlet valve controlling the flow of the steam to the turbine.